1. Field of the Invention
The present invention relates to a new design of an optical head capable of providing a subwavelength beam.
2. Description of the Related Art
Optical lithographic technology has been broadly used in various researches due to its convenience since 1665. Besides, since the middle of 20th century, the related applications are deeply extended to various high technology industries, for example, semiconductor and optical storage industries (e.g. CD, DVD etc.) However, owning to the diffraction limit, various optical applications confront with same difficulties when an optical resolution smaller than one wavelength is required.
Optical Lithography
Under the push of Moore's law of the semiconductor industry, the optical etching linewidth has been shrunk from 5 micrometers in the late 1960 to 90 nanometers nowadays. The optical etching linewidth is still persistently shrunk. Since the visible light optical etching fulfills advantages of high yield and low cost, it is always a primary etch technique in semiconductor processes. Because the dimension of the diffraction limit is equivalent to the wavelength, it is difficult to further shrink the optical etching linewidth when the linewidth reaches up to the order of the wavelength. For the sake of persistently shrinking the etching dimension, the development of short wavelength light sources has become an important field to study. The light source has been varied from 436 nm visible wavelength to 248 nm deep ultraviolet wavelength and till 157 nm nowadays. The light source with the shorter wavelength, even more X-ray range, is still developed.
The shrinkage of the exposing wavelength reduces the size of a focusing optical spot. However, the optical elements suitable for the visible light range are not light transmitted in the short wavelength range. Only fused silica and less material are suitable for the ultraviolet range. The flexibility of selection of the optical materials is significantly reduced. Moreover, the refractive index of the above materials in the short wavelength range is not high. It is quite difficult to design an appropriate lens with high numerical aperture and low aberration. The requirement of the accuracy of a phase mask used during exposing is getting stricter because the exposing wavelength becomes shorter. Besides, owing to the property of wave propagation of laser light in free space, the depth of focus and focusing optical spot have the same dimension. As a result, when the focusing optical spot approximates to a sub-micrometer size, the depth of focus would approximate to surface roughness of a general test sample. Therefore, it is necessary to add a fast automatic focusing system to correct the optical path to avoid the defocusing phenomenon that arises an unexpected optical spot, when performing etching. To summarize the foregoing, the optical mechanism becomes more complicated when performing etching as the wavelength shrinks, and the cost is increased more and more.
On the other hand, although the current non-optical etching method can provide a higher space resolution, it cannot provide the property of high yield of the optical etching method. To give an example by the electron beam lithographic technique, which utilizes electron beams composed of accelerated electrons to impact the material, resulting in chemical or physical reactions to attain the effect of etching patterns. Since the material wavelength of the electron is far smaller than the wavelength of light, its diffraction limit is smaller and the resolution can attain several nanometers. However, the equipment is very expensive and needs to operate in vacuum, and the yield thereof is also limited. Hence, the equipment is not suitable for being as a parent machine for manufacturing products in large quantities. The non-optical etching method is mostly used in the preparation of original masks. In addition, there is a new lithographic technique as called atomic force microscopy lithography developed in recent years. The atomic force microscopy lithography utilizes a probe of the atomic force microscope to generate electric field to cause an inducing selective chemical reaction, for example etching or deposition. The atomic force microscopy lithography provides a high resolution of ten-nanometer order, but its etching area is too small and the etching speed is too slow. To summarize the foregoing, the optical lithography is still un-replaceable for the manufacturing process with high yield.
Optical Storage
As a non-contact property of the optical method, the optical storage provides the following advantages: 1. non-destructive by abrasion; 2. long life time; and 3. non-influence by dust when reading. Moreover, the optical storage device has a high optical storage density. The application of the optical storage is widespread. For example, CD (Compact Disc) and DVD (Digital Video Disc) have become indispensable data storage media in modern life. As the rapid advancement of network, multimedia and software, it is a trend to develop a data storage media with a higher capacity and a smaller volume.
The present commercialized optical storage devices include CD, DVD and MO (Magnetic Optical device). Since DVD-ROM (Digital Video Disc-Read Only memory) provides a higher capacity and a capability for reading CD-ROM, it has replaced CD-ROM in recent years. Although MO is directed to a storage system with a high capacity and high speed, it cannot become a main stream in the marketing due to its highly cost.
The optical storage device usually writes data in a compact disc, and its recording method is through indentations with different lengths between the tracks of the compact disc. The intensity of the light reflected from the indentations is weaker and the intensity of the light reflected from the tracks is stronger. Thus, by way of detecting the intensity of the light reflected from the compact disc to read data recorded therein. The compact discs of CD-ROM (Compact Disc-Read Only Memory) and DVD-ROM (Digital Video Disc-Read Only Memory) are produced in large quantities by copying the data recorded in the mold by pre-pressing. Nevertheless, CD-R and DVD-R utilize a laser source with a short wavelength to break the long chain of dye molecules to change the refractive indexes so as to form low-reflective indentations to write data. Phase change material is applied to CD-RW, DVD-RW and DVD-RAM, and which uses a high-power laser with short pulses to write data, by which the phase change material is rapidly cooled to form an amorphous state, which has a lower reflective index than that of the crystalline state formed by annealing with a long-pulse laser, thus to form indentations. The tracks of the compact disc are formed of a saw-teethed structure having peak and valley portions so as to conveniently write into data along the tracks. Except for the DVD-RAM capable of recording data in both of the peak and valley portions for improving data density, remaining optical storage devices record data in the valley portions.
For the optical pickup head, a laser spot is focused unto a surface of the compact disc through an objective, and reflected from the surface of the compact disc to image on a light detector through the objective. The resolution of the optical pickup head is confined by the size of the optical spot. When focusing the light source, the size of the optical spot is mainly relied upon a result gotten by dividing the wavelength λ of the light source by the numerical aperture of the objective. The size of the optical spot on the surface of the compact disc is determined by the multiplication of the thickness d of a substrate of the compact disc and the numerical aperture. Making a comparison, the pitch of the tracks of DVD is 0.74 μm, the shortest length of the indentations of DVD is 0.43 μm, a laser light with λ 650 nm and NA (Numerical Aperture) 0.6 can be used to access the compact disc of DVD; the pitch of the tracks of CD-ROM is 1.6 μm, the shortest length of the indentations of CD-ROM is 0.83 μm, a laser light with λ 780 nm and NA (Numerical Aperture) 0.45 can be used to access the compact disc of CD-ROM.
In order to obtain high storage density, it had better have a unit storage area as small as possible. However, due to the diffraction limit, the size of the focusing optical spot of the optical pickup head at the best can approximate to the wavelength of the light source. As a consequence, the unit storage area cannot be further shrunk. It is currently a trend to shrink the wavelength of the light source. There are many difficulties exiting in the technology using a light source with a short wavelength. Meanwhile, the depth of focus become shallower and requirement of stability of the compact disc is improved, resulting in a significant increase of the cost.
Optical Imaging and Probing
The resolution of the far-field optical measuring system is confined by the principle of the diffraction. Waves with too high space frequency become evanescent waves, and cannot propagate to far field. Thus, the optical spot cannot be focused to a spot less than the wavelength order, and the resolution only can reach up to about the wavelength. Near-field optical microscope is a kind of surface monitoring instrument that can break through the diffraction limit of the conventional optical microscope. The near-field optical microscope generally associates with a voltage actuator or an air bearing to form a system to perform the height-feedback control. Therefore, the optical probe can be accurately controlled over the surface of the sample to be monitored at a height about several to hundreds nanometers. When performing three-dimensional feedback-controllable near-field scanning, surface topography and optical image can be obtained, and the resolution can reach up to about 30 nm to 100 nm. The optical fiber probe is often used as the probe, and the diameter of its tip is between 50 nm and 100 nm.
Synge in the United Kingdom in 1928 and O'keefe in the United States in 1956, respectively propose the basic principle of the near-field optical microscope, which utilizes a distance far less than a wavelength to perform optical measurement to break through the diffraction limit. E. A. Ash and G. Nicholls of the UCL university of the United kingdom firstly completes the experimental verification of the near-field optical microscope, which utilizes microwave with a 3 cm wavelength to pass the microscope formed of a probe with a 1.5 mm aperture, and a 0.5 mm resolution is readily obtained. And, a space resolution about 1/60 wavelength can be obtained in the near field. Bell laboratory utilizes optical fiber as a probe by a shear-feedback control method in 1992 to complete a first near-field optical microscope. By way of shrinking the aperture of the probe and the distance between the probe and surface of the object to be monitored to obtain a smaller focusing optical spot and information of evanescent waves unavailable by the far-field optical microscope, thus breaking through the diffraction limit. The near-field optical microscope provides a quite high space resolution in measuring a testing object, providing another definite and practicable method for measuring a micro object.
However, there are many limitations existing for the near-field optical microscope: for detecting evanescent waves, an approximating zero working distance between the probe and the surface of the testing object is required, and to obtain the approximating zero working distance, a precise feedback control technology and an expensive air-bearing machine are required. On the other hand, since the light transmittance is too small, it is not easy to obtain a good signal to noise ratio. If the intensity of the incident light is to be increased, the tip of the probe is easily destroyed since the temperature is over high.
Extraordinary Transmittance Phenomenon Caused by a Surface Subwavelength Structure
Dr. Ebbesen proposed an extraordinary transmittance phenomenon caused by a surface subwavelength structure in Nature in 1998, which cannot be explained by the conventional diffraction phenomenon. The light transmittance measured by experiments is far higher than the result calculated by the micro-hole diffraction theory proposed by Bethe in 1944, and arising many discussions and studies. FIG. 1 shows important parts of a series of studies made by the team of Dr. Ebbesen, in which it is discovered that the light transmittance through the subwavelength hole arrays perforated a metal layer and a underlying substrate is far higher than that calculated by the conventional diffraction theory. A subsequent study indicates that the extraordinary transmittance phenomenon still happens if there is a periodic structure formed on the surface of the metal layer as an auxiliary, and it is not necessary for the hole-arrayed structure to perforate the metal layer and substrate. Besides, it is discovered that a structure of concentric circles with a central perforated hole can improve the light transmittance. Dr. ebbesen et al. publish another important article in August, 2002 that a subwavelength structure is formed on each face of a metal thin layer, and improving the light transmittance and the divergence angle of the transmitting light is far smaller than that predicated by the diffraction theory. For example, in case that groove period=500 nm, groove depth=60 nm, hole diameter=250 nm, film thickness=300 nm, it is discovered that the energy of the light beam (λpeak=660 nm) transmitting the hole-arrayed structure is confined within 3 degree. It shows that the hole-arrayed structure makes the transmitting light beam have directionality, which is totally contrary to the perception of the conventional optics that when the light beam is incident in a hole smaller than the wavelength of the light beam, the transmitting light beam would provide isotropous divergence, i.e. viewing the hole approximating to a point light and the outward propagating waves as spherical waves.